1. Field of the Invention
The present invention relates to a pressure reducing valve. In particular, the present invention relates to a pressure reducing valve changing an output pressure in correspondence to an engine speed.
2. Description of the Related Art
The following means has been employed in a conventional construction vehicle such as a wheel loader, a bulldozer or the like.
A delivery flow amount of a pump is changed in correspondence to an operation amount of a pilot operating valve. The pilot operating valve reduces a pressure of a supplied pressurized oil in accordance with the operation amount and outputs the oil. In the case that it is intended to change the pump discharge flow amount in correspondence to the engine speed, the pressure of the pressurized oil supplied to the pilot operating valve is changed.
A hydraulic circuit mentioned above is shown, for example, in Japanese Laid-Open Patent Publication Nos. 49-71353 and 10-122363.
FIG. 5 shows an embodiment of a hydraulic circuit having the same function as that of a hydraulic circuit changing the supplied pressure to the pilot operating valve in correspondence to the engine speed, which is disclosed in the publication mentioned above. A variable displacement type pump 1xe2x80x2 is connected to various kinds of actuators, for example, a traction motor of a vehicle, a cylinder and the like which are not illustrated. A displacement controller 4xe2x80x2 of the variable displacement type pump 1xe2x80x2 is connected to an output side of an output circuit 6xe2x80x2 in an operating apparatus 5xe2x80x2. A fixed orifice 8xe2x80x2 is provided in a delivery pipe passage of a fixed displacement type pump 2xe2x80x2. An upstream pressure of the fixed orifice 8xe2x80x2 is increased in correspondence to the engine speed.
Accordingly, a differential pressure between front and rear of the fixed orifice 8xe2x80x2 becomes larger in correspondence to the engine speed.
In this case, the pressure reducing valve inherently has a function of making an output pressure uniform. In the normal pressure reducing valve, the output pressure of the pressure reducing valve is made uniform due to a balance between an output side pressure P1xe2x80x2 and a spring 9axe2x80x2 provided in the pressure reducing valve. When the balance of the force is changed, the output pressure is also changed. In this case, the pressure reducing valve can not, of course, output a pressure more than the supplied pressure.
The pressure reducing valve 7xe2x80x2 shown in FIG. 5 applies the differential pressure between front and rear of the fixed orifice 8xe2x80x2 so as to change the output pressure, in addition to the balance. That is, an upstream pressure P2xe2x80x2 of the fixed orifice 8xe2x80x2 is applied in a direction of a position (A)xe2x80x2 in FIG. 5 so as to change the balance in such a manner as to increase the output pressure of the pressure reducing valve. A downstream pressure P3xe2x80x2 of the fixed orifice 8xe2x80x2 is applied in a direction of a position (B)xe2x80x2 so as to change the balance in such a manner as to reduce the output pressure of the pressure reducing valve. The differential pressure between front and rear of the fixed orifice 8xe2x80x2 is changed on the basis of the engine speed. That is, the force applied in the direction of the position (A)xe2x80x2 is increased in accordance with an increase of the engine speed, and the output pressure of the pressure reducing valve 7xe2x80x2 becomes large.
In the hydraulic pressure shown in FIG. 5, when the engine speed is increased, the pressure P1xe2x80x2 supplied to the operating valves 6axe2x80x2, 6bxe2x80x2 is increased. Accordingly, when the engine speed is increased, the maximum pressure output by the operating apparatus 6xe2x80x2 is increased.
The output circuit of the operating apparatus 6xe2x80x2 has a pair of pilot operating valves 6axe2x80x2 and 6bxe2x80x2. Each of the pilot operating valves 6axe2x80x2 and 6bxe2x80x2 is a pressure reducing valve, and changes the output pressure in correspondence to an amount of incline of the operating lever 5axe2x80x2. The output pressure is zero at an illustrated neutral position. When the operating lever 5axe2x80x2 is tilted in a direction of a, the pressure in correspondence to the amount of incline of the operating lever 5axe2x80x2 is output via the pilot operating valve 6axe2x80x2. At this time, in another pilot operating valve 6bxe2x80x2, the output pressure keeps zero.
The displacement controller 4xe2x80x2 changes the displacement of the variable displacement type pump 1xe2x80x2 in correspondence to the output pressure of the operating apparatus 6xe2x80x2. In the hydraulic circuit shown in FIG. 5, it is designed such that the displacement of the variable displacement type pump 1xe2x80x2 becomes maximum at the maximum output pressure of the operating apparatus 6xe2x80x2 in a certain great area or more of the engine speed. Accordingly, when the engine speed is increased, the maximum displacement that the variable displacement type pump 1xe2x80x2 can operate is increased.
FIG. 6 shows a change of the output pressure of the pressure reducing valve 7xe2x80x2 in correspondence to the engine speed.
However, in the reducing control valve mentioned above, since the pressure supplied to the operating apparatus 6xe2x80x2 is small in a low rotational speed area of the engine, the pressure P5xe2x80x2 output to the displacement controller 4xe2x80x2 is also small even when the operating lever is largely operated. Accordingly, the displacement of the variable displacement type pump 1xe2x80x2 is small and there is generated a case that the displacement is less than a necessary flow amount in the actuator.
An example is shown here. In the construction vehicle such as the wheel loader, the bulldozer or the like mentioned above, a plurality of working machines or traveling apparatuses are operated by the operating apparatus 6xe2x80x2. When the engine speed is reduced for achieving a low speed travel, and the pressure P1xe2x80x2 supplied to the operating apparatus 6xe2x80x2 is reduced in correspondence thereto so as to reduce the displacement of the variable displacement type pump 1xe2x80x2, there may be a case that a flow amount of the traveling apparatus which does not require a large flow amount due to a low speed travel is sufficient but a flow amount applied to the working machine performing a work without relation to the traveling speed is insufficient.
Further, although an illustration is omitted, there is a requirement that a command pressure applied to the traveling apparatus of the vehicle is intended to be sufficiently secured for securing a vehicle speed and a traction force even in the case that the engine speed is reduced when the output pressure of the operating apparatus 6xe2x80x2 shown in FIG. 5 is used as the command pressure applied to the traveling apparatus of the vehicle. At this time, there may be a case that the command pressure can not be sufficiently secured in the hydraulic circuit employing the pressure reducing valve 7xe2x80x2 shown in FIG. 5.
An object of the present invention is to provide a pressure reducing valve which can reduce an engine speed and secure a minimum pressure.
In accordance with a main aspect of the invention, there is provided a pressure reducing valve consisting of a pressure reducing valve portion whose output pressure is uniform and a differential pressure responding portion comprising springs changing the output pressure of the reducing valve portion. Spring forces of the springs are adjusted by differential pressure between front and rear of a fixed orifice provided at a delivery pipe passage of a fixed displacement pump whose rotational speed is in correspondence to an engine speed. In this pressure reducing valve, the output pressure of the pressure reducing valve portion is changed in correspondence to an engine speed by the spring forces of the springs applied to a side in the pressure reducing valve portion to which output pressure of the pressure reducing valve portion is applied.
Consequently, in the pressure reducing valve changing the output pressure in correspondence to the rotational speed of the engine, the output pressure does not become equal to or less than a predetermined pressure at a rotational speed equal to or less than the first predetermined rotational speed even when the engine speed becomes low. Accordingly, it is possible to optionally set a target minimum force. Further, it is possible to provide the pressure reducing valve which can change the output pressure in correspondence to the engine speed at the first predetermined rotational speed or more.
Preferably, in the pressure reducing valve, a spring force of the spring is applied to a side opposite to the side in the pressure reducing valve portion to which the output pressure of the pressure reducing valve portion is applied.
As a result, in the same manner as that of the main aspect, in the pressure reducing valve changing the output pressure in correspondence to the rotational speed of the engine, the output pressure does not become equal to or less than a predetermined pressure at a rotational speed equal to or less than the first predetermined rotational speed even when the engine speed becomes low. Further, the output pressure does not become equal to or more than a predetermined pressure when the engine speed is over the second predetermined rotational speed larger than the first predetermined rotational speed. Accordingly, it is possible to optionally set a target minimum pressure and maximum pressure. Further, it is possible to provide the pressure reducing valve which can change the output pressure in correspondence to the engine speed in a range equal to or more than the first predetermined rotational speed and equal to or less than the second predetermined rotational speed.